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Aggregator with group by and "First" / "Last" option in informatica should give you the same result as Decode statement on database side. Performance betterment is still doubtful. You can do the same in a expression transform by checking current and previous rows for subject area change and allot values to ports. After that you can use a ran transform and use top option or aggregator to get you intrested rows to output. Leaving it for your thinking.